Sample and test scripts for this matrix is at github repo. Let me know if you find any mistakes (it must include some!) and my misunderstandings.

This matrix is a part of my presentation Oedo HTTP client picture scroll at Oedo RubyKaigi01(in Japanese) hosted by Asakusa.rb yesterday. You can see other comparisons including dependency, timeline and performance in the presentation. There are some Japanese commentary in it but almost major parts are in English and images.

[EDIT]

Disclaimer: I'm the author of httpclient gem, which is one of a clients in the list. :)